The invention relates to a hot-water appliance capable of resisting at least the pressure of the public water supply system, comprising at least one hot-water vessel with a supply conduit connectable to the public water supply system and a discharge conduit connectable to a draw-off tap. The hot-water vessel further comprises a heating element contained in the hot-water vessel, temperature regulation and a substantially cylindrical outer jacket wall and preferably with two end walls.
Such a device, which, in that case, is intended to supply water of substantially 100xc2x0 C., is known from British patent No. 1,373,990. The known device is provided with a plastic foam heat insulation. A drawback, more and more felt in the last few years from environmental considerations, of this type of frequently used devices having a hot-water vessel as buffer reservoir, which is often continuously maintained at higher temperatures, is the heat loss. This is particularly true of devices intended to frequently and immediately supply small amounts of hot water. The solution for the heat loss has hitherto been sought in the improvement of the insulating material and the use of a greater layer thickness. Both approaching methods give insufficient results for small hot-water appliances of at most 20 liters capacity, which are intended for heating temperatures above at least 80xc2x0 C. In practice, it has been found hardly possible to obtain affordable, much better insulating properties than, for instance, those of a high-quality polyurethane foam, for which, from considerations of energy saving, a layer thickness of 4 cm is advisable. The use of this greater layer thickness of the insulating jacket, however, does not lead to the desired result, because devices for immediately supplying small amounts of hot or boiling water require that the heating vessel is placed as close to the draw-off point as possible to prevent time loss, water loss, and energy loss owing to cold lead through cooling of the intermediate pipe between the draw-off point and the heating vessel. Close to the draw-off point, such as, for instance, in the kitchen of a household in the kitchen cabinet under the draining board close to the sink, much too little space is usually present, however, to enable the arrangement of a hot-water appliance having a sufficiently thick insulating jacket. It is therefore highly important that the outside dimensions of a heating appliance for this kind of applications are as small as possible. The arrangement of a hot-water appliance under a washbasin, close to the warm-water tap, also urgently requires a smallest possible dimension, while retaining a sufficient water capacity of sufficiently high temperature, to open up a large market segment of energy saving.
The total insulating layer of a small cylindrical hot-water vessel of less than 20 liters capacity occupies much space when compared to the water volume. Take, for instance, a small upright cylindrical reservoir having a height/diameter ration of 2/1, then of a diameter of 12.4 cm the capacity is 3 liters. When this cylinder is covered at the side wall and at the end faces with an insulating layer thickness of only 3 cm, the total capacity is already more than 8 liters. In this case an insulating volume of more than 5 liters is required to insulate 3 liters with an insulating thickness which, from environmental considerations, should actually be more than 4 cm for appliances that are switched on day and night. This is all the more true of the use of the pertinent type of hot-water appliances in air-conditioned spaces.
European patent application EP-A-0 309 198 describes a hot-water device comprising a hot-water vessel, which hot-water vessel is insulated by means of a vacuum insulating jacket. The publication, however, very clearly described that in such a device it is undesirable that the heating means are contained in the tank, because this considerably increases the production cost of the device in connection with the opening which has to be present in the vacuum insulating jacket. Moreover, this publication states that through the heating by means of a heating element arranged in the vessel a mixing of cold water with added warm water and warm water already present in the tank occurs, so that it is impossible after drawing off an amount of hot water to immediately provide hot water of a desired temperature. The European publication therefore proposes to arrange the heating element outside the tank and to design it as an instantaneous heating element. A drawback of this solution is of course that the heat loss occurs at the instantaneous heating element, because this instantaneous heating element is not insulated. If such an instantaneous heating element is to be insulated, this would be done by means of insulating foam, which, in turn, would lead to undesirable large dimensions. Moreover, the heating coil has a much higher temperature than the liquid, which substantially complicates the selection of the insulating materials. In the device according to the present invention this problem has been solved by still arranging the heating elements in the tank, in spite of the attendant problems, and insulating, with another form of insulation, only the opening in the vacuum insulating jacket through which the heating means still extend into the tank.
WO A 85/01790 relates to a solar boiler comprising a vacuum insulating jacket. This publication does not teach a person skilled in the art anything more than that insulation can be effected with a vacuum insulating jacket. Furthermore, the publication is not relevant to the present invention, since it does not disclose the arrangement of a heating element in the hot-water vessel. Moreover, the known device is not provided with a temperature regulation, and the tank is not of cylindrical, but of spherical design. It is not clear how the transparent spherical shell halves of the outside jacket in the known device can be connected together such that a vacuum can be created therein which is maintained for a longer period. Moreover, no indication whatever can be derived from the publication for the height of the vacuum. The information that the vacuum is almost 100% is meaningless to a person of average skill in the art. The connection between the inside tank and the outside shell at the location of the passage of the conduits is not further explained either and forms a position susceptible to leakage.
The American patent U.S. Pat. No. 4,974,551 relates to a water heater made of plastic. It is true that it is described therein that the container is insulated by means of a vacuum insulating jacket, but, in practice, plastic is absolutely unsuitable for the performance of a sealing function. Moreover, in the course of time, plastic itself releases a large number of gases which remove the vacuum in the jacket. Therefore, in this known device the vacuum required for insulation is absolutely not present. The publication therefore proposes to insulate the jacket with insulating material, such as glass wool or urethane foam. In the device known from the American patent a vacuum of ca. 10-2 mb, as proposed in a further elaboration of the present invention, is absolutely impractical.
U.S. Pat. No. 3,830,288 relates to an insulating jacket for a heat storing device which is heated with inexpensive current, and which, during the day, releases its heat to the space in which it is arranged. Preferably, these heat storing devices are located below the window and are therefore of flat and rectangular design. This publication therefore does not relate to a hot-water appliance comprising a hot-water vessel with a supply conduit which is connectable to the public water supply system and a discharge conduit which is connectable to a draw-off tap. Moreover, the insulating jacket described in this publication is filed with gas. This is contradictory to the proposal according to the invention in which a high vacuum is proposed for the insulation of the hot-water vessel.
Non of the above-discussed publications therefore discloses a hot-water appliance comprising a vacuum insulating jacket. Even less do these publications disclose a hot-water appliance the insulating jacket of which is of such design that the heat loss per unit area of surface area to be insulated is not more than 200 watts per m2 at a temperature difference between the inner space enclosed by the insulating jacket and the ambient space of at least 90xc2x0 C. and at a thickness of the insulating jacket of at most ca. 2 cm. Such a degree of insulation can, as described above, be obtained according to a further elaboration of the invention, because the pressure in the insulating jacket being under a vacuum is less than 10-2 millibar.
The invention relates to a compact hot-water appliance with a very high degree of insulation characterized in that at least the cylindrical wall part of the hot-water vessel is insulated with a vacuum insulating jacket.
When using a vacuum insulating jacket which covers at least the cylindrical jacket wall of the hot-water vessel, it has been found possible to use insulating wall thicknesses of, for instance, 1 cm or even thinner, with better insulating properties than 4 cm thick polyurethane foam. When a thick conventional insulating layer is used for one or even for both end faces of the cylinder, it is surprising to see how much smaller external volume of the total heating reservoir can be obtained and how strongly the heat losses can be reduced in practice, even when using water temperatures above 100xc2x0 C., by at least insulating the cylindrical wall part of the hot-water vessel with a vacuum insulating jacket. It is preferred here if, according to a further elaboration of the invention, the insulating jacket is of such design that the heat loss per unit area of surface area to be insulated is not more than 200 watts per square meters at a temperature difference between the inner space enclosed by the insulating jacket and the ambient space of at least 90xc2x0 C. and at a thickness of the insulating jacket of not more than ca. 2 cm.
To reach such an insulating value with a vacuum insulating jacket having a thickness of at most 2 cm and preferably ca. 1 cm, a high vacuum with an internal pressure less than ca. 10xe2x88x921 millibar, preferably ca. 10xe2x88x923 millibar or even less, is advisable.
According to a further elaboration of the invention, it is very favorable if the cylindrical wall part of the hot-water vessel is insulated with a vacuum insulating jacket, the inner and outer walls of which are connected together at the location of at least one connecting edge, which connecting edge is situated at a distance from the water in the hot-water vessel which is greater than the distance between the inner and outer walls at the location of the hot-water vessel, while the distance between the connecting edge and the hot-water vessel is bridged by an insulating jacket wall part. The effect thus obtained is that the insulating jacket wall part forms a heat bridge between the hot-water vessel being at high temperature and the outer wall of the vacuum insulating jacket being at ambient temperature. The heat resistance of this heat bridge can be increased by increasing the height of the insulating jacket wall part, by reducing the material thickness of the insulating jacket wall part and by selecting a material for the insulating jacket wall part having a low heat conductivity. Thus the unavoidable heat losses as a result of conduction can be strongly reduced.
Furthermore, it is very favorable if the or each connecting edge defines an opening in the insulating jacket which gives access to the hot-water vessel. Thus the exchange of, for instance, heating elements and the removal of scale become possible, while, moreover, if desired, a passage is provided for inlet and outlet openings.
To limit as much as possible the wall part of the hot-water vessel not insulated by the vacuum insulating jacket, it is very favorable if the height/diameter ratio of the hot-water vessel is at least 1.5/1.